1. Field of the Invention
This invention relates to the measurement of chromatic dispersion of optical components and, more particularly, to the measurement of chromatic dispersion of narrow band optical components having a large dispersion variation in a narrow transmission band.
2. Description of the Related Art
As is known and understood, chromatic dispersion is an important optical parameter which affects optical transmission system performance. For such reason, those high data rate systems which have been deployed and which are presently being developed require a detailed dispersion characterization of the various optical components in the transmission path. Because some of the components (such as narrow band filters) have considerably high chromatic dispersion to the optical signal wavelengths, a dispersion characterization of such components becomes an important consideration of the design engineer. Although component dispersion measuring electronic instruments have been described (see, for example, U.S. Pat. No. 5,033,846 --Hernday et als), those commercial dispersion test sets are especially intended for determining chromatic dispersion in single mode optical fibers. Attempts to utilize such electronic instruments for measuring chromatic dispersion in optical components, however, have generally not provided accurate measurement, especially where the components have a large dispersion variation in a narrow transmission band. Illustrative of these components are narrow band optical filters as employed in higher channel count DWDM (Dense Wavelength Division Multiplex) systems.
Currently, two available commercial test sets to measure chromatic dispersion of optical components are based on a group delay measurement technique--with the dispersion being calculated from the group delay data. The first, more popular, reliable and accurate technique employs delay curve fitting and requires group delay data as a function of wavelength in determining an analytic function that is fitted to the data; dispersion values are then obtained by taking a derivative of the function with respect to the wavelength. (However, because this technique requires the obtaining of group delay data over a wide wavelength range to obtain reasonable accuracy, the technique has been determined somewhat less than ideal for narrow band optical component measurements.) The second, less popular technique calculates dispersion from group delays at two adjacent wavelengths--without having to measure the group delay data at many wavelengths as with the curve fitting approach. (But, with this technique, a difficulty resides in trying to obtain accurate dispersion values when the wavelength and the group delay differences are either very small or very large.)
Other chromatic dispersion measurement techniques are also available--such as an Interferometric Method, in which a relative phase of signals from two optical paths is used, one from the device under test (DUT) and one from a reference; and a Time Domain Measurement Method, in which actual pulse width changes are measured in the time domain due to transmission through the test sample for known spectral widths. In an alternative Swept Frequency Technique, on the other hand, dispersion is calculated from the interference pattern of two different wavelengths; in one manner employing several lasers to generate amplitude variation due to a phase difference between each of the test signals and a reference signal, with the group delay data for dispersion calculation being obtained from the amplitude variation due to the interference--while in a second manner, a tunable laser is employed to generate two modulation side bands by modulating a light source at various wavelengths, with the dispersion being directly calculated from the interference pattern.
While these chromatic dispersion measurement techniques have proved useful for dispersion measurements of optical fibers in which the dispersion does not vary rapidly over a wide wavelength range, they have been found to be less than desirable for optical component measurements where there exist a very high dispersion variation within narrow transmission bands--and because the group delay data for dispersion calculation are difficult to obtain when the phase change is very large for a small optical wavelength change. In those instances, significant measurement errors arise.